1. Field of the Invention
The present invention relates to an ink jet printing apparatus that carries out printing using thermal energy and a method of controlling the temperature of a head of the ink jet printing apparatus, and in particular, to improvements in control of the head temperature in a low-temperature environment.
2. Description of the Prior Art
It is known that an ink jet printing apparatus or the like is subjected to various adverse effects of a variation in environmental temperature or in temperature of a head composed of integrated printing elements. This is because the temperature varies physical property values such as the viscosity or surface tension of ink. Further, with what is called a bubble jet (registered trade name) printing method of using thermal energy to generate bubbles in ink in order to eject the ink, a variation in temperature may vary the conditions under which bubbles are generated.
If these physical property values of ink and the bubble generation conditions vary, the amount of ink droplets ejected from a printhead or the accuracy of landing may vary, resulting in a variation in density, a nonuniform density, or a variation in tone.
Accordingly, for ink jet printing apparatuses, it is important to control the temperature of the head. A conventional method of controlling the temperature of the printhead is described in U.S. Pat. No. 5,861,895 and Japanese Patent Application Laid-Open No. 5-220964 (1993). This control method employs a configuration that uses a heater for heating the printhead (a heater exclusively used to control temperature or used both for ink ejection and for temperature control) and a temperature sensor detecting temperature related to the printhead to feed back the temperature detected by the temperature sensor so as to adjust the amount of heat generated by the heater. Another conventional method does not use such feedback control but provides open loop control such that the heater is regulated to achieve an arbitrary preset temperature.
Such methods of controlling the heater of the printhead are roughly classified into four types: methods of always adjusting the head temperature (using feedback control based on a detected temperature), methods of adjusting the head temperature at fixed time intervals (using feedback control based on a detected temperature), methods of adjusting the head temperature when it exceeds an environmental temperature (using feedback control based on a detected temperature), and methods of modulating the pulse width of a heat pulse.
Of these conventional temperature control methods, a known one detects the head temperature at the start of printing or during every printing operation for one line, and compares the detected temperature with a reference temperature to provide such control that the printhead is heated until a target temperature is reached if the detected temperature is lower than the reference temperature. In this case, a fixed upper limit is generally set for the heating time in order to limit a decrease in throughput associated with the heating operation to below the fixed value.
Furthermore, as disclosed in U.S. Pat. No. 5,168,284 and U.S. Pat. No. 5,475,405, the head temperature may be controlled in real time by comparing the head temperature with the reference temperature and adding a non-printing pulse to the head on the basis of a difference between the head temperature and the reference temperature.
Moreover, U.S. Pat. No. 6,260,940 discloses a technique of preheating the printhead during sheet feeding or during an acceleration or deceleration period of the printhead.
U.S. Pat. No. 5,861,895, mentioned previously, discloses a technique of varying the waveform of a drive signal on the basis of the head temperature to suppress a variation in amount of ink ejected from the printhead, the variation attributed to the head temperature, while reducing a self temperature increase.
Further, Japanese Patent Application Laid-open No. 5-220965 (1993) discloses a technique of using ejecting heating means (heater) to heat the printhead up to a first temperature and using subheating means having a subheater to heat the printhead up to a second temperature higher than the first temperature.
Furthermore, Japanese Patent Application Laid-Open No. 5-96718 (1993) discloses a technique of heating, if a plurality of transporting means are provided, the printhead using timing corresponding to a transporting operation of each transporting means.
Moreover, with a higher grade of an image to be printed, the adverse effects of a variation in density or tone associated with the head temperature are more serious. U.S. Pat. No. 5,477,246 discloses a technique of providing such control that the temperature is maintained depending on the type of printing in order to vary the amount of ink ejected from the head depending on whether the type of an object is a character or an image.
However, the above control is disadvantageous in that throughput decreases substantially owing to the heating time set for the head. In particular, if a text or the like using black, with which the temperature of the printhead does not increase significantly (self-increase in temperature) during a printing operation, is printed using a multipath printing process, the throughput decreases further markedly when the printhead is heated for every print line. To avoid this, it has been envisaged that an upper limit is set for the heating time. However, in this case, printing is executed with the printhead insufficiently heated. In particular, this tendency appears clearly in a portion of the image printed immediately after the start of printing, when a self temperature increase is small. Further, problems such as a nonuniform density are prone to occur. Accordingly, this method is improper for high-grade image printing.
Furthermore, to heat the printhead during printing in order to avoid a decrease in throughput, a printing pulse and a heating pulse must be individually controlled, thereby requiring the apparatus to be complicated. On the other hand, if the heating operation period is limited to the period in which no printing operation is performed, i.e. a sheet feeding period or a carriage acceleration or deceleration period, then insufficient heating may be provided during the heating operation period as with the case in which the upper limit on the heating time is set to a smaller value as described previously. Further, during the heating operation period, motors for the transporting system and carriage are accelerated or decelerated to increase power consumption. Thus, disadvantageously, if the printhead is to be further heated, the capacity of a power supply must be increased.
Further, it has been proposed that heat retention be executed depending on the type of printing by controlling heat retention when a noticeable high-grade image, which may create a problem as described above, is printed. However, in this case, advanced determining means is required which can automatically determine the image to be printed.
Furthermore, if it is determined whether or not the printhead is to be heated on the basis of the head temperature, the temperature of ink may vary with the environmental temperature even with the same head temperature. As a result, the behavior of ejection may vary. To avoid this, it has been envisaged that temperature detecting means is provided which measures the environmental temperature in addition to the head temperature. However, this increases costs and requires correction of a difference between the ink temperature and the environmental temperature caused by heat generated by the printing apparatus itself. Therefore, the required control is complicated.